Internal combustion engines represented by, for instance, a diesel engine are used in various fields because of their high combustion efficiency and low generation of carbon dioxide. However, because of emissions of internal combustion engines contain nitrogen oxides (hereinafter referred to as “NOx”), and because of recent increases in environmental consciousness, NOx emission control has a tendency to be strengthened.
NOx emitted from a marine diesel engine is scheduled for Tier III controls starting in 2016 by the International Maritime Organization (IMO). Under such Tier III controls, NOx will be required to be reduced 75% from values of Tier II controls currently in effect. For this reason, development of NOx removal technology that is highly effective and inexpensive is required, and various investigations about the reduction of NOx are being performed (see Non-Patent Literature 1).
A specific example of NOx reduction technology based on emission after-treatment includes selective catalytic reduction (SCR) technology for selectively reducing NOx in emissions of a diesel engine using a catalyst (see Non-Patent Literature 1). However, the SCR technology requires chemicals such as urea or ammonia to reduce NOx, and has a challenge in the management of the chemicals or excessive costs. Further, when the SCR technology is applied to emissions having a high concentration of NOx from several hundreds to thousands of ppm, increase in size of the device is also expected to be a problem.
As a specific example of NOx reduction technology based on combustion control, a method of supplying supply air that has been humidified by spraying water to an internal combustion engine and reducing NOx is proposed (see Non-Patent Literature 2). In this method, a concentration of oxygen in the supply air can be efficiently reduced using an effect of diluting water vapor. However, since a pressure of the water vapor is a function of only a temperature, when a supercharging pressure is increased at the same temperature, there is a drawback that an amount of humidification is relatively reduced. Further, if a temperature of the supply air is increased too much in order to increase the amount of humidification, a problem of reduction of fuel efficiency may occur.
As another method of controlling combustion using water vapor, a method of transferring the water vapor from emissions to supply air using a water vapor permeable membrane is proposed (Patent Literature 1). In this method, since an amount of the transferred water vapor is small, there is a drawback that a NOx reduction effect is low.
In addition, a method of reducing NOx using a membrane is proposed. For example, it is described in Patent Literature 2 that a concentration of nitrogen in the air is enriched using an oxygen permselective membrane, and the nitrogen-enriched air is supplied to an internal combustion engine to reduce NOx.
Furthermore, a method of adding water to fuel and supplying the emulsion to an engine, or directly injecting water into an engine cylinder is also proposed (Non-Patent Literature 1). This method has an advantage in that a NOx reduction effect according to water per unit weight is high. However, NOx cannot be reduced up to a Tier III level by this method alone.
Further, a technique (exhaust gas reduction (EGR)) for causing some emissions to circulate in supply air, and reducing a concentration of oxygen in the supply air in order to reduce NOx in the emissions has been developed (Non-Patent Literature 1). The emissions contain SOx that is generated by oxidation of sulfur in the fuel, and becomes sulfuric acid when absorbed into water in the emissions. In EGR, to avoid sulfuric acid corrosion of a machine pipe at a portion at which the emissions circulate, a desulfurization or scrubber waste water treatment facility based on a water scrubber of the emissions is required. For this reason, the device is complicated, and there is a problem in the aspects of initial device expenses, operating costs, and maintenance costs.